Existing communication systems suffer from a number of problems which act to degrade the quality of communication between a transmitting unit and a receiving unit within the communication system. In particular, in a radio telephone network for example, which comprises a plurality of mobile stations or radio telephones communicating with a base station connected to other base stations in the radio telephone network, both the radio telephone and the base station are relatively close to ground level. Since the base station and radio telephone are close to ground level obstacles such as buildings, walls, cars and people hinder the direct line of sight between them especially in urban areas. Thus, they typically communicate by reflected or diffracted radio wave signals. Due to the multiple reflections and diffractions the r.f. power received by a radio telephone or a base station is at a much lower level than would be expected from the inverse square law if direct line of sight communication was possible. Typically, the power loss is of the form d.sup.-.alpha. where d is the distance between the transmitting and receiving stations and .alpha. lies between 3 and 4. This power loss is known as path loss.
The problem of path loss has been addressed in known radio telephone systems by the base stations monitoring the strength of signals received from various radio telephones communicating therewith and from time to time issuing a request over the air for an individual radio telephone to increase or decrease its transmitting power. The radio telephone responds by adjusting the gain of its transmitting amplifier which is typically under microprocessor control. Generally, the amplifier is operable at one of a plurality of predetermined output power levels which are selected automatically in response to the request from the base station for a change in the level of the output power. Typically, the power levels are defined in the radio telephone system specification. For each power level a nominal value is specified together with a permitted tolerance range.
In addition to the normal fading there is another form of fading known as Rayleigh fading. This type of fading is a short term fading and is characterised by rapid variations in the r.f. power level of a signal received by a radio telephone or base station. It is caused by the multiple signal paths arising from the reflections and diffractions forming a quasi-stationary standing wave pattern with nulls at approximately half wavelength intervals of the signal frequency.
The effect of the periodic nulls in received signal power due to Rayleigh fading is that transmitted data may be lost thereby introducing errors into the transmission of data. In order to ensure that there is sufficient integrity in the radio telephone network redundant data has to be sent such as error-correcting codes. This results in a reduced information or data handling capacity for the network.
Heretofore, the problems of Rayleigh fading have been addressed by using a technique known as Slow Frequency Hopping (SFH) or Frequency Hopping. In this technique, the carrier frequency of a particular communication channel is discontinuously changed between discrete carrier frequencies of a set of defined carrier frequencies. Since the Rayleigh fading of signals at different frequencies is not the same, and becomes increasingly different as the difference between the frequencies increases, frequency hopping for a particular communication channel substantially reduces the effects of Rayleigh fading for that communication channel effectively transforming errors due to Rayleigh fading into widely spread random errors. Another advantage is that co-channel interference from other cells is reduced.
In the known GSM system for cellular radio telephony, the sequence of data bursts making up a particular communication channel or Traffic CHannel (TCH) are cyclically assigned to different frequencies by the base station handling that communication channel. Additionally, a technique known as interleaving is employed in the GSM system. This involves jumbling up data to be transmitted such that normally adjacent groups of data are transmitted at different times, and de-interleaving the transmitted signal at the receiver.
A disadvantage of a system using frequency hopping is that the "memory" of the radio propagation channel is lost. This so-called "memory" arises because a radio propagation channel can be considered a time-variant linear filter. Such a filter has a well-defined auto-correlation function, and there is a limit to the variation in a signal level between two closely spaced points (of the order of a half wavelength). If a signal strength is known at one point then with a defined confidence interval it can be predicted at a next closely spaced point. Thus, the radio propagation channel can be considered to have a "memory".
Loss of the memory of a communication channel results in accurate and fast power control being difficult to achieve. Furthermore, frequency hopping does not remove the generation of r.f. signal nulls and overcoming such nulls by fast power control cannot be achieved since there is no memory in the communication channel and the occurrence of nulls cannot be predicted. Additionally, frequency hopping requires relatively complex circuitry both in the base stations and radio telephones in order to ensure that the correct frequency is used at each time. In particular, the frequency synthesizers and the Tx and Rx circuitry are complex. There must also be a substantial number of carrier frequencies for the communication channels to jump to and this may not always be possible in a crowded r.f. spectrum. Also, it is possible that two or more different communication channels will simultaneously land on the same frequency which will cause gross interference.